Current renewable energy sources, such as wind and solar, only provide energy intermittently and generally do not coincide with peak load times. Thus, large scale energy storage would be important for more efficiently utilizing renewable energy sources and integrating them into the grid. A proposed solution for energy management is to develop smaller, smarter grids with localized energy storage capabilities, so that any excess energy generated may be stored and then reintroduced to the grid during peak load times. A particular technology capable of storing large amounts of electrical energy is the redox flow battery (RFB). This type of battery, also called a semi-fuel cell, uses liquid solutions of redox-active chemicals as energy storage media, rather than the solid-state electrode materials found in conventional batteries. In an RFB, energy invested by means of an external supply of electric current and voltage is converted into electrochemical potential energy by directing opposite redox reactions in the anolyte and catholyte. The stored electrochemical energy can be converted into electrical energy upon discharge with concomitant reversal of the opposite redox reactions.
While most batteries contain solid electrodes, RFBs are powered by electroactive species dissolved in liquid electrolyte solutions, i.e., a catholyte and an anolyte, which are stored in large tanks and flowed through parallel plates between current collectors and an ion selective membrane. The modular design of RFBs allows for the independent control of power and energy density by controlling such parameters as flow rate, catholyte/anolyte concentration, electrode surface area, and storage tank size.
The RFB was initially developed by Thaller, et al. working for the U.S. National Aeronautics and Space Administration (U.S. Pat. No. 3,996,064). The Thaller RFB system was based on Fe3+/2+ and Cr3−/2− redox couples in acidic solution. More recently, there has been great interest in the development of all-vanadium RFB systems, which use four different redox states of vanadium to facilitate two redox reactions in the battery full cell (e.g., Skyllas-Kazacos, M., et al., J. Electrochem. Soc., 1987, 134, pp. 2950-2953; and Skyllas-Kazacos, M., et al., J. Electrochem. Soc., 2011, 158, R55). Notably, the same chemical parent species (vanadium) is used for both of the requisite redox reactions, i.e., vanadium ions in both the catholyte (V IV/V) and the anolyte (V III/II), which minimizes the effects of membrane crossover. However, the vanadium RFB suffers from a low cell voltage (1.26 V, compared to 3.6 V in Li-ion batteries) and therefore, limited energy density. This low voltage and solubility lead to a maximum energy density of typically about 25 Wh/L for current technologies.
Other RFB systems are known, such as a bromine-polysulfide system (U.S. Pat. No. 4,485,154), zinc-bromine system (Lex, P., et al., Power Eng. J., 1999, 13, 142-148), and bromine-anthraquinone aqueous RFB system (Huskinson, B., et al., Nature, 2014, 505, 195-198). However, these other RFB systems generally suffer from membrane crossover issues, low effective molarities, and lower than optimal cell voltages, volumetric capacities, and energy densities.